Cecidomyiidae, gall midges, is a family encompassing about 5000 species (Gagné 1989) of which several are important noxious insects within the agricultural area all over the world. One of these gall midges is C. nasturtii, which infests Brassica vegetables, in particular Brussels sprouts, cauliflower and broccoli. The female C. nasturtii lays its eggs in the tips (buds) of cruciferous host plants, where the larvae feed and develop. Feeding damage near the growing tips induces distortion and gall-like symptoms. When the larvae have become fully developed they will go down into the soil and form cocoons, in which they will pass into the chrysalis stage and then either emerge to start the next generation cycle, or begin a diapause, which can be one or two winters long. In springtime, the larvae will leave the winter cocoon and create a summer cocoon in which it will develop to chrysalis and then to an adult midge. The emergence is determined by temperature and moisture. Mating is thought to occur shortly after emergence. Both sexes are known to leave the emergence field (due to crop rotation usually not a cruciferous crop) and once mated females will look-for a field with a Brassica crop to lay their eggs.
One reason for high damage levels is the lack of a good monitoring method for determining the time of emergence The (monitoring) method used today means sifting and floatation of soil samples, which are then analyzed under microscope to determine in which development stage the chrysalises are. The analyses are carried out by someone skilled in the art and are very time consuming. Soil samples of a few fields are, however, only analyzed. As the emergence is dependent on the local climate (moisture and temperature) the accuracy of (the monitoring) becomes low with regard to other fields. Furthermore, the adults appear very suddenly and for a short time period, and as the control actions must be carried out prior to egg laying, the method is very uncertain. It requires a very efficient communication system as well, in order to reach all farmers with the results of the prognostication (monitoring) and recommendation concerning the time for controlling action. Currently, the only available monitoring method is based on the exposition of yellow pan traps filled with water and a detergent. The traps do not catch species-specifically, but rather all phytopagous insects are attracted and, therefore, the analysis of the trap content is laborious and the tiny swede midges can be detected and identified only by well trained personnel. Furthermore, the trapping efficiency of yellow traps is low for the swede midge and, with rather low populations, the crop is frequently damaged by C. nasturtii even when no midges are detected in traps. Temperature-based forecasting methods have been developed but failed to give accurate, field-specific prognosis.
In pheromone based monitoring methods a limited number of traps baited with a pheromone related to the intended insect, in particular a sexual pheromone, in synthetic form, are used. A pheromone trap is the most sensitive monitoring system known today. Using such, even very low population densities can be detected. Since pheromones are highly specific with respect to the attracted insect species, traps baited with such pheromones catch predominantly the targeted species, which greatly facilitates the analysis of the traps content and the detection of the respective species.
Noxious insects, as well as all other insects, can vary a lot in number from year to year and their occurrence during the season may vary too. The difficulty is thus to know if and when they need to be controlled. The trap catches will, however, provide an answer to three essential questions, viz: 1) if the insect is present in the field; 2) when the insect is present in the field; and 3) approximately how many there are. The answer to the first question is essential to prevent or eliminate unnecessary control treatments as a matter of precaution, using insecticides. Having the answer to question no. 2), an optional control treatment can be made at the right time, i.e. when the insect is there.
Pheromone baited traps do not provide an exact measure of the population density but the catch result denotes if there is a low, an average, or a high risk infestation. Pheromone based monitoring systems are used today for about 200 different noxious insects, in particular moths. With such monitoring traps the use of insecticides can be lowered with 50 to 75%. Unnecessary control treatment can thus be avoided, which not only reduces the farmer's costs but also limits the adverse effects on the environment Should the trap catches indicate that control treatments are necessary, one will be able to carry out the control at the most sensitive development stage of the insect.
Chemical insecticides to noxious insects will, with regard to the impact on the environment and their quite often broad control spectrum (i.e. they target many different insects), be replaced by biological control methods. Such methods utilize basic knowledge of the ecology and biochemistry of the specific insects. The precision, i.e., the desired species specificity can thus become very high. The methods, which are the commercially most successful ones, are based on the sexual pheromones or other pheromones of insects. The use of pheromones in insect pest management is based on the use of naturally occurring compounds at naturally occurring amounts and concentrations; which means maximum consideration of the environment.
When controlling noxious insects with sexual pheromones one imitates and utilizes the natural situation in which a female attracts a male by emitting the sexual pheromone, which will be spread as a mist in the wind direction. A male who comes into the pheromone plume responds by flying against the wind towards the female. For control purposes, a large number of synthetic pheromone sources are applied in an area that will thus be flooded with pheromone. In this scent environment the female's own signal will be drowned and the male will have no possibility to find her. The technique is called mating disruption and is a commonly spread control method.
It has been shown that females use a sexual pheromone in order to attract the males in approximately 10 gall midge species (for a review, see Harris and Foster, 1999; Hillbur, 2001). However, chemical identifications of pheromones have so far only been made in the pea midge, C. pisi (Hillbur et al. 1999, 2000, 2001), the orange wheat blossom midge, Sitodiplosis mosellana (Gries et al. 2000), and the Douglas-fir cone gall midge, C. oregonensis (Gries et al. 2002).